1. Field of the Invention
This invention relates to a process for the aftertreatment of betaines and amphoteric surfactants in which the surfactants are alkalized and subjected to a thermal aftertreatment.
2. Statement of Related Art
Betaines and amphoteric surfactants show high compatibility with the skin and exhibit excellent cleaning properties. Accordingly, they are particularly suitable for the production of a number of surface-active products. In the most simple case, they are produced from tertiary amines which are reacted with sodium chloroacetate to form alkyl betaines. The reaction of fatty acid amidoamines or imidazolines with sodium chloroacetate leads to the formation of amphoteric surfactants of the glycinate type. If acrylates are used as the alkylating agent, aminopropionates are formed. Compounds of the type mentioned are described in a number of synoptic articles of which it is only intended here to cite Parf. Cosm. Arom. 70, 67 (1986), HAPPI, 70, (November 1986) and Soap Cosm. Chem. Spec. 46, (April 1990).
A particular concern in the production of betaines and amphoteric surfactants is to provide pure and hence dermatologically and toxicologically safe products. For example, traces of chloroacetic acid and, more particularly dichloroacetic acid, in the surfactants are undesirable. Preservatives which are intended to protect the betaines and amphoteric surfactants against microbial contamination are also frequently undesirable, so that there is a further need for products which are stabilized against microbial contamination even without the addition of auxiliaries. Finally, a third problem addressed by the present invention was to provide light-colored products.
A number of publications offering partial solutions to the cumulated problem are known from the prior art.
For example, it is proposed in DE-A1 39 39 264 (Henkel) to reduce the content of chloroacetic acid in amphoteric surfactants by subsequent treatment of the aqueous solutions with ammonia, amino acids or oligopeptides. DE-OS 29 26 479 (Th. Goldschmidt) describes a process in which the quaternization is carried out at a pH value of 7.5 to 10.5 and the residual content of free alkylating agent is thus reduced. The teaching of DE-A 20 63 424 (Rewo), which describes pH adjustment for the alkylation of imidazolines, points in the same direction. However, these processes do not have any influence on the content of dichloroacetic acid.
Sodium hypochlorite and, more particularly, hydrogen peroxide are used to bleach surface-active compounds. Bleaching is generally carried out in a neutral or acidic medium because H.sub.2 O.sub.2 quickly decomposes under alkaline conditions. However, the acidic peroxide bleaching of betaines and amphoteric surfactants often produces only a temporary lightening of color, the color darkening again after storage.
In addition, a process for the preservative-free stabilization of special nonionic surfactants, so called alkyl polyglucosides, in which the aqueous solutions are adjusted to a pH value of at least 7, is known from the literature (DE-A1 40 35 722, Henkel). However, in view of the differences in structure between the nonionic alkyl polyglucosides and the betaines or amphoteric surfactants, which in addition also contain a hydrolyzable amide bond, application of the process to betaines and amphoteric surfactants appears to offer little promise.